Method for the production of hyperpolarized 129Xe

ABSTRACT

The present invention relates to a method for the production of hyperpolarized  129 Xe and to a method for the production of a contrast agent.

This application is a filing under 35 U.S.C. 371 of internationalapplication number PCT/NO2003/000352, filed Oct. 24, 2003, which claimspriority to application number 20025124 filed Oct. 25, 2002, in Norwaythe entire disclosure of which is hereby incorporated by reference.

The present invention relates to a method for the production ofhyperpolarized ¹²⁹Xe and to a method for the production of a contrastagent.

¹²⁹Xe is a gas at room temperature. The nucleus has a spin quantumnumber of ½, and a moderately large nuclear magnetic moment of −1.347494nuclear magnetons. It can be taken up into the lungs and absorbed intoblood or tissue. It has been recognized that it has potential to beimaged in the body via magnetic resonance imaging (MRI). However, sincethe gas phase is approximately 1000 times less dense (in moles/liter)than the condensed phase of biological material (e.g. blood, tissue),its nuclear magnetic resonance (NMR) signal is much weaker than that ofthe protons in the condensed biological material. To surmount this,hyperpolarized ¹²⁹Xe has been prepared. In this case, the nuclearmagnetization, upon which the MRI sensitivity depends, can be increasedby 5 orders of magnitude, making the contrast available with the ¹²⁹Xeeven in the gas phase larger than that from the protons in theirequilibrium room temperature condensed phases. Because the spin is ½,the retention time of the non-equilibrium highly polarized state of thehyperpolarized ¹²⁹Xe, frequently referred to as the spin-latticerelaxation time T₁, is long enough even at body temperature for the¹²⁹Xe to persist in the hyperpolarized state for sufficient time toobtain contrast enhanced MR images. Thus, hyperpolarized ¹²⁹Xe gas hasgenerated considerable interest as an inhalable contrast agent formagnetic resonance imaging of the lungs.

W. Happer et al., Phys. Rev. A29, 3092 (1984) described the productionof hyperpolarized ¹²⁹Xe using optical pumping laser techniques. Adisadvantage of this method is the low production rate, due topolarization being achieved in the low density gaseous phase. Thus, onlyrates of a few liters per hour are achievable.

WO-A-99/35508 discloses hyperpolarization of xenon in the solid stateusing the “brute force” method or the dynamic nuclear polarization (DNP)method.

WO-A-00/23797 discloses additional methods for the hyperpolarization ofxenon in the solid state, such as doping xenon with paramagnetic oxygenmolecules, irradiating the xenon with ionizing radiation or thedispersal of magnetized small particles encapsulated in polymers whichare placed in the xenon.

It has now surprisingly been found that the presence of an additive inDNP hyperpolarization of xenon in the solid state dramatically increasespolarization enhancement.

The present invention provides a method for producing hyperpolarized¹²⁹Xe comprising

-   -   a) preparing a mixture of xenon, an additive and a free radical    -   b) hyperpolarizing said mixture according to the DNP method to        obtain hyperpolarized ¹²⁹Xe and    -   c) optionally separating said xenon from the other components of        the mixture.

In a first step a) a mixture of xenon, an additive and a free radical isprepared.

According to the invention, xenon can be used in its natural form, i.e.a mixture of several isotopes including ¹³¹Xe (21.2%) and ¹²⁹Xe (26.4%).Alternatively, ¹²⁹Xe enriched xenon can be used.

The term “additive” according to the invention encompasses also suitablemixtures of additives. Preferably, at least one solvent or a mixture ofsolvents is used as an additive in the method according to theinvention. More preferably, at least one solvent or a mixture ofsolvents is used which has good glass-forming properties, e.g. singlechained alcohols like ethanol or propanol or glycols and/or lipophilicproperties, e.g. like toluene or methylcyclohexane. Further preferredare solvents or mixtures of solvents which contain a high amount of NMRactive nuclei such as ¹H, ¹⁹F, ³¹P and the like. Particularlypreferably, the additive is at least one solvent selected from the groupconsisting of straight chain or branched C₆-C₁₂-alkanes,C₅-C₁₂-cycloalkanes, fatty alcohols, fatty esters, substituted benzenederivatives like toluene or xylene, mono- or polyfluorinated solventslike tetradecafluorohexane or hexafluoroisopropanol, single chainedalcohols like ethanol, propanol or butanol and glycols. Most preferredadditives are cyclopentane, toluene, xylene, ethanol, propanol and2-butanol.

In a preferred embodiment, the additive is chosen as such that there isa temperature/pressure region where both the additive and Xenon aresimultaneously in the liquid state. Both propanol and ethanol aresuitable examples of such additives.

In a further preferred embodiment, the amount of xenon in the mixture ofxenon and additive is kept low, as the obtained ¹²⁹Xe polarizationdecreases when the concentration of xenon in the mixture of xenon andadditive is increased. However, since the intensity of the NMR signal isdetermined by both polarization (which increases with dilution) and thenumber of ¹²⁹Xe spins (with decreases with dilution), these two factorshave to be balanced when choosing the amount of xenon for the DNPpolarization.

The free radical in the mixture of step a) may either be a stable freeradical such as a nitroxide or a trityl radical or a free radicalprepared in situ from a stable radical precursor by a radical-generatingstep shortly before the hyperpolarization step b), or alternatively bythe use of ionising radiation. Suitable free radicals are organic freeradicals such as triarylmethyl, nitroxide radicals such as porphyrexide,TEMPO, TEMPONE and TEMPOL (see below), oxygen centered radicals such asgalvinoxyl (see below), carbon centered radicals such as trityls andallyls, metal ions with unpaired electrons such as Cr(V), e.g.BHHA-Cr(V) and EHBA-Cr(V) (see below), Mn(II), e.g. MnCl₂, Tm(II),Yb(III), Nd(III), V(IV), Ni(II) and Fe(III) ions or radiation generatedradical centers and biradicals, e.g. those described in WO-A-88/10419,WO-A-90/00904, WO-A-91/12024, WO-A-93/02711 and WO-A-96/39367. Preferredfree radicals are those which dissolve in the additive and/or in liquidXenon. Particularly preferred free radicals are trityls and nitroxideradicals, e.g. tert.-amyl-tert.-butyl nitroxide.

In a preferred embodiment, xenon gas is condensed on top of the additiveand free radical in a suitable reaction vessel, preferably by using aliquid nitrogen bath. The reaction vessel is subsequently sealed andwarmed up until the components are in the liquid state. The additive andthe free radical are mixed with the liquid xenon until a homogeneousmixture is obtained. The formation of a homogeneous mixture may beachieved by several means known in the art such as agitation, shaking,stirring and the like. The resulting mixture is then cooled rapidly,e.g. in a liquid nitrogen bath, and the solid obtained is used for thehyperpolarization.

In a second step b), the mixture of step a) is hyperpolarized accordingto the DNP method to obtain hyperpolarized ¹²⁹Xe.

Suitably, the mixture will be cooled, e.g. in liquid nitrogen, in orderto result a solid which can be used for the DNP hyperpolarization.

DNP mechanisms include the Overhauser effect, the so-called solid effectand the thermal mixing effect. During the DNP process, energy, normallyin the form of microwave radiation, is provided. There is a transfer ofpolarization from the unpaired electron of the radical to ¹²⁹Xe and/orthe NMR active nuclei of the additive, depending on the properties ofthe free radical and/or the frequency of the microwave radiationapplied. If the NMR active nuclei of the additive are polarized, thispolarization may be transferred to ¹²⁹Xe subsequently by a suitablecross-polarization sequence. The DNP method may utilize a moderate orhigh magnetic field and a very low temperature, e.g. by carrying out theDNP process in liquid helium and a magnetic field of about 1 T or above.The temperature should be very low, e.g. 100 K or less, preferably 4.2 Kor less, more preferably 1.5 K or less, especially preferably 1 K orless and even more especially preferably 100 mK or less. The magneticfield strength used should be as high as possible, suitably higher than0.1 T, preferably higher than 1 T, more preferably 5 T or more,especially preferably 15 T and more and most preferably 20 T and more.Alternatively, a moderate magnetic field and any temperature at whichsufficient enhancement is achieved may be employed. Preferably, thepolarization should 1% or more, more preferably 10% and more, especiallypreferably 25% and more and most preferably 50% and more.

After hyperpolarization xenon may be separated from the other componentsof the mixture by simply warming the mixture until xenon is in a gaseousstate and collecting the gas in a suitable container. Warming of themixture can be achieved by different means such as contacting themixture with a hot liquid like water, or using laser or microwave energyto melt the mixture. Such means for dissolving and meltinghyperpolarised solid samples are described in WO-A-02/37132 andWO-A-02/36006. Optionally, the obtained xenon gas can be condensed againto obtain “xenon ice” which can be transported using a permanent magnetand a liquid nitrogen bath. Preferably, the magnetic field strength forsuch a transport should be as high as possible, suitably 10 mT or more,preferably 0.1 T or more, more preferably 0.2 T or more and especiallypreferably 0.3 T or more. The temperature for such a transport should bebelow the boiling point of xenon, i.e. below 166.05 K at atmosphericpressure.

For the use as a contrast agent, the condensed xenon may conveniently beheated prior to its use.

Thus, another aspect of the invention is a method for the production ofa contrast agent comprising

-   -   a) preparing a mixture of xenon, an additive and a free radical    -   b) hyperpolarizing said mixture according to the DNP method to        obtain hyperpolarized ¹²⁹Xe    -   c) separating xenon from the other components of the mixture,        and    -   d) optionally condensing the separated xenon again.

Yet another aspect of the invention is the use of DNP-hyperpolarized¹²⁹Xe for the manufacture of a contrast agent for the use in magneticresonance imaging of the human or non-human animal body, preferably ofthe lungs of the human or non-human animal body.

Yet another aspect of the invention is a method for magnetic resonanceimaging of the lungs of a human or non-human animal body comprising

-   -   a) preparing a mixture of xenon, an additive and a free radical    -   b) hyperpolarizing said mixture according to the DNP method to        obtain hyperpolarized ¹²⁹Xe    -   c) separating said xenon from the other components of the        mixture,    -   d) optionally condensing and heating said separated xenon    -   e) administering said xenon to the lungs of a human or non-human        animal body and    -   f) generating magnetic resonance images of said body.

Yet another aspect of the invention is the use of ¹²⁹Xe which has beenhyperpolarized according to the method of the invention as a contrastagent, more preferably as a contrast agent for magnetic resonanceimaging of the lungs.

EXAMPLES Example 1 Comparison Example

10 μl of tert.-amyl-tert.-butyl-nitroxide in a reaction vessel werecooled in a liquid nitrogen bath. 750 ml of gaseous xenon (naturalabundance ¹²⁹Xe, STp (=standard temperature and pressure)) werecondensed into the reaction vessel. The reaction vessel was sealed andthe temperature was adjusted to 195 K. The content was agitated until ahomogeneous liquid was formed and then cooled down in a liquid nitrogenbath. The reaction vessel and the liquid nitrogen bath were then movedto a N₂-glove box. The reaction vessel was opened and liquid nitrogenwas added. The solid content of the reaction vessel was pulverized witha spatula and transferred to a pre-cooled sample holder. The sample wasthen rapidly inserted into a cryostat and DNP polarization was performedusing a magnetic field of 3.35 T, an irradiation frequency of 93.3 GHzand a temperature of 1.6 K.

T₁ was measured to ca. 10 h at 1.6 K and 3.35 T. No DNP effect wasobserved.

Example 2 Comparison Example

Example 2 was carried out as Example 1 using 100 μl oftert.-amyl-tert.-butyl-nitroxide. T₁ was measured to ca. 1 h at 1.6 Kand 3.35 T. No DNP effect was observed.

Example 3

Example 3 was carried out as Example 1 using 10 μl oftert.-amyl-tert.-butyl-nitroxide in 1.2 ml toluene and 800 ml of gaseous¹²⁹Xe. DNP polarization was performed using a magnetic field of 3.35 T,an irradiation frequency of 93.3 GHz and a temperature of 1.44 K. Apolarization enhancement of 24 was measured at 1.44 K and 3.35 T,corresponding to a polarization of ¹²⁹Xe of 1.6%.

Example 4

Sample:

1.5 ml propanol, 26 mgTris-(8-ethoxycarbonyl-2,2,6,6-tetralis-(methylbenzo[1,2-d:4,5-d′]bis(1,3)dithiole)methyl,in the following named “radical”, 500 ml (STP) natural abundance xenon.

Description of Experiment:

The Radical and propanol were inserted into a round bottom flask thatwas subsequently the flask evacuated from air and flushed with heliumgas several times to reduce the contents of oxygen in the system. Theflask was then immersed in a liquid nitrogen bath and xenon gas wasallowed to condense into the flask. After sealing the flask, the liquidnitrogen bath was replaced by an ethanol/CO₂ bath. The content of theflask was agitated by magnetic stirring. The ethanol/CO₂ bath was thenreplaced by an ethanol bath and cooled to 163 K using liquid nitrogen.At this temperature both propanol and xenon are in the liquid phase andthe content of the flask was a viscous liquid. Additional magneticstirring was performed followed by rapid cooling in a liquid nitrogenbath. The flask was opened and liquid nitrogen was added. The obtainedsolid content of the flask was pulverized with a pre-cooled spatula andtransferred to a pre-cooled sample holder. The sample was rapidlyinserted into a cryostat and DNP polarization was performed using amagnetic field of 3.354 T, an irradiation frequency of 93.93 GHz (200mW) and a temperature of 1.08 K.

Results:

The obtained DNP enhancement was a factor of 82 compared to the thermalequilibrium signal, which corresponds to a polarization equal to 7.2%.The time constant for polarization build-up was 1.2 hours, and the T₁was estimated to be 4.2 hours.

Example 5

Sample:

3.85 ml propanol, 52 mg Radical, 500 ml (STP) natural abundance xenon(corresponding to 0.85 ml liquid xenon).

Description of Experiment:

The experiment was performed in the same way as Example 4.

Results:

The obtained DNP enhancement was a factor of 263.4 compared to thethermal equilibrium signal, which corresponds to a polarization equal to23.2%. The time constant for polarization build-up was 2.2 hours, andthe T₁ was estimated to be 4.6 hours.

Example 6

Sample:

1.0 ml propanol, 20.5 mg Radical, 500 ml (STP) natural abundance xenon.

Description of Experiment:

The experiment was performed in the same way as Example 4.

Results:

The obtained DNP enhancement was a factor of 26 compared to the thermalequilibrium signal, which corresponds to a polarization equal to 2.3%.The time constant for polarization build-up was 1.2 hours, and the T₁was estimated to be 2.5 hours.

Example 7

Sample:

3.85 ml propanol, 52.7 mg Radical, 500 ml (STP) ¹²⁹Xe-enriched xenon(82.3% ¹²⁹Xe).

Description of Experiment:

The experiment was performed in the same way as Example 4.

Results:

The obtained DNP enhancement was a factor of 197 compared to the thermalequilibrium signal, which corresponds to a polarization equal to 17.4%.The time constant for polarization build-up was 1.7 hours, and the T₁was estimated to be 6.2 hours.

Example 8

Sample:

3.85 ml ethanol (99.5%), 52.2 mg Radical, 500 ml (STP) natural abundancexenon.

Description of Experiment:

The experiment was performed in the same way as Example 4.

Results:

The obtained DNP enhancement was a factor of 171.6 compared to thethermal equilibrium signal, which corresponds to a polarization equal to15.2%. The time constant for polarization build-up was 4.1 hours, andthe T₁ was estimated to be 4.4 hours.

Example 9

Sample:

3.85 ml 2-butanol, 51.4 mg Radical, 500 ml (STP) natural abundancexenon.

Description of Experiment:

The experiment was performed in the same way as Example 4.

Results:

The obtained DNP enhancement was a factor of 23 compared to the thermalequilibrium signal, which corresponds to a polarization equal to 2.0%.The time constant for polarization build-up was 1.5 hours, and the T₁was estimated to be 3.9 hours.

Example 10

Sample: Same preparation as in Example 5.

Description of Experiment:

The initial part of the experiment was performed as in Example 5, exceptthat the irradiation frequency was 93.945 GHz. The sample was polarizedfor 2 hours and subsequently thawed in situ using hot water (≈95° C.).The xenon gas was collected in a bag normally used for storage ofhyperpolarized helium gas. The xenon gas was then transferred into a 10mm NMR tube which had been pre-filled with argon. The NMR tube wassealed with a cap, and transferred to a 9.4 Tesla NMR spectrometer fordetection.

Results:

The DNP enhancement in the solid state was not determined. The timeconstant for polarization build-up was approximately one hour. Theobtained polarization enhancement in the gas phase was a factor of 4752compared to the thermal equilibrium signal at room temperature, whichcorresponds to a polarization equal to 4.3%.

1. A method for producing hyperpolarized ¹²⁹Xe comprising a) preparing a mixture of xenon, at least one solvent or a mixture of solvents selected from the group of single chain alcohols glycols, toluene, cyclopentane and methylcyclohexane, and a free radical b) hyperpolarizing said mixture according to the DNP method to obtain hyperpolarized ¹²⁹Xe and c) optionally separating said xenon from the other components of the mixture.
 2. A method according to claim 1, wherein the mixture in step a) is prepared from liquid xenon.
 3. A method according to claim 1, wherein the mixture in step a) is prepared by condensing xenon gas on the top of the at least one solvent or mixture of solvents and the free radical, warming the components until xenon and the at least one solvent or mixture of solvents are in a liquid state and mixing the components until a homogeneous mixture is obtained.
 4. A method according to claim 1, wherein in step b) ¹²⁹Xe is directly hyperpolarized.
 5. A method according to claim 1, wherein in step b) the NMR active nuclei of the at least one solvent or mixture of solvents are hyperpolarized and this polarization is subsequently transferred to ¹²⁹Xe by a cross-polarization sequence.
 6. A method according to claim 1, wherein xenon enriched with ¹²⁹Xe is used.
 7. A method according to claim 1, wherein in step c) xenon is separated from the other components of the mixture by warming the mixture until xenon is in the gas state and collecting said xenon in a suitable container.
 8. A method for the production of a contrast agent comprising a) preparing a mixture of xenon, at least one solvent or a mixture of solvents selected from the group of single chain alcohols, glycols, toluene, cyclopentane and methylcyclohexane, and a free radical; b) hyperpolarizing said mixture according to the DNP method to obtain hyperpolarized ¹²⁹Xe c) separating said xenon from the other components of the mixture, and d) optionally condensing the separated xenon again. 